Advanced laser systems have a plurality of mirrors in the beam train. Misalignment can occur to produce pointing errors, particularly when the laser system operates in a nonvisible wavelength. In such systems, in order to achieve alignment, a beam at an alignment wavelength which is different than the principal wavelength can be used for alignment purposes. When the principal wavelength and alignment wavelength are different, pointing errors can occur because of nonuniformities in standard high reflectance coatings. When the beam at alignment wavelength reflects off a standard high reflectance plural coated mirror for the principal wavelength, interference effects cause the alignment beam to have an optical path difference which varies nonlinearly with the coating thickness. This nonlinear response causes distortions in the alignment beam at the alignment wavelength and, hence, causes pointing errors.
In order that the beam at alignment wavelength properly permits alignment of the optical system for the principal wavelength, a mirror, which is efficient and accurate for both the principal wavelength and the alignment beam wavelength, is necessary.
It is understood that Perkin-Elmer has created a multiple layer coating designed for reflection of plural wavelengths. The Perkin-Elmer design is specifically for use in a carbon dioxide laser resonator cavity and includes a visible reflector to align the mirrors, together with an infrared stack for the carbon dioxide laser infrared radiation. The substrate carries the infrared stack, and on top of that is a nickel layer. On top of the nickel layer is a stack of layers for the visible reflector. In the Perkin-Elmer design the nickel layer absorbs over quite a large band, including the carbon dioxide infrared laser frequency. The metal layer is incorporated to produce an etalon effect at 9.27 micrometers which gives a sharp reflectance peak. The coating was designed to lower the gain except at the wavelength of interest. The metal layer does not affect the reflectance or the optical path difference at the alignment wavelength. High absorption in the metal layer causes mirror damage at lower energy levels.